import tensorflow as tf

#创建4个张量
a = tf.constant(1.)
b = tf.constant(2.)
c = tf.constant(3.)
w = tf.constant(4.)

with tf.GradientTape() as tape: #构建梯度环境
    tape.watch([w]) # 将w加入梯度跟踪列表
    # 构建计算过程
    y = a * w**2 + b * w + c

#求导
[dy_dw] = tape.gradient(y,[w])
print(dy_dw)